Polymer Chemistry Blog

Sardon’s group investigate the ring opening polymerization of cyclic esters using novel photo acid generators.

Light has emerged as a powerful stimulus allowing for spatial and temporal control over polymerization kinetics, macromolecular sequence, and composition and has enabled a number of high-end applications including coatings, microelectronics, additive manufacturing and 3D printing. However, the potential of light in polymer chemistry is far from being fully exploited. Photopolymerization is currently dominated by (controlled) radical polymerizations of vinyl monomers. Little attention has been paid to photo-induced cationic ring-opening polymerization (CROP) of cyclic esters. In a recent contribution to Polymer Chemistry, Sardon and co-workers developed six new photocatalysts for light-mediated CROP. Upon exposure to light, the new photocatalysts release strong sulfonic acids that can trigger the CROP of ε-caprolactone. The authors particularly focused on imino-sulfonates and aryl-sulfonates based photocatalysts, and this strategy was hugely successful. Complete monomer conversion was obtained after only 5 minutes of irradiation. This is an impressively high polymerization rate despite the catalyst efficiency being typically strongly related to the chromophore and the sulfonate substituent. In addition, several of these photocatalysts are stable even at 100 °C and were successfully used to produce not only linear biodegradable polymer polymers but also crosslinking poly(ε-caprolactone) exhibiting excellent mechanical properties. Furthermore, the authors employed density functional theory calculation to propose a photodissociation mechanism. The studied photopolymerization has also been successfully applied to surface coating. The potential applications of these new photocatalysts are certainly not limited to photocoating, and therefore, we look forward to seeing further exciting applications of these photocatalysts.

Tips/comments directly from the authors:

• The efficiency of the PAGs was observed to be highly dependent on the chromophore and the photolabile bond; imino-sulfonates were more capable of producing sulfonic acids than aryl-sulfonates. However, their preparation is tedious and requires a multistep synthesis process. Aryl-sulfonates are not as efficient but their synthesis is performed in a single step in excellent yields up to 90%.

• Imino-sulfonate based photocatalyst were able to promote the ring opening polymerization of ε-caprolactone at room temperature but 3 h were required for getting full conversion. Nevertheless, as several of these catalyst were stable up to 100 °C we were able to get full conversion in just 5 minutes at 100 °C

• To further expand the applications of the studied PAGs, we demonstrated the ability of the photoacid to promote the crosslinking at room temperature in the presence of a dilactone. While the crosslinking reaction was successful, long reaction times were required for reaction completion, making impractical the use of these photocatalyst in 3 D printing applications.

Citation to the paper: Novel imino- and aryl-sulfonate based photoacid generators for the cationic ring-opening polymerization of ε-caprolactone, Polym. Chem., 2021,12, 4035-4042, DOI: 10.1039/D1PY00734C

Link to the paper:

https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00734c#!divAbstract

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

Rossegger et al. employ a photolatent catalyst for the local activation of topological rearrangements in thermo-activated vitrimers.

Vitrimers are covalent adaptable polymers networks which have recently attracted tremendous interest thanks to their unique feature of switching from a classic thermoset behaviour to a malleable plastic upon heating. In particular, at low temperature, vitrimers exhibit properties similar to a thermoset (e.g. rigid, brittle, opaque, high strength, good chemical resistance, etc.). Instead, heating vitrimers to temperatures above their topological freezing temperature, leads to activation and exchange of the covalent bonds within the networks thereby allowing the polymer chains to flow like viscoelastic liquids. However, one of the main limitations of this thermoresponsive feature is the lack of spatial control. In their current contribution, Schlögl and coworkers report a novel photocatalyst that can introduce spatial control to vitrimers. In particular, triphenylsulfonium phosphate was used as a photocatalyst to release strong Brønsted acids in a vitrimer region exposed to UV light (365 nm). The acids subsequently catalyse the bond exchange of vitrimer networks only in this local UV-exposed region, thus fully controlling the vitrimeric property. Furthermore, this new chemistry was not only confirmed by stress relaxation studies but was also applied to develop shape-changing vitrimer materials. Importantly, the triphenylsulfonium phosphate catalyst is stable at high temperatures and transparent in the visible light region. As such, visible light (405 nm) could be used to prepare the vitrimer in 3D structures without introducing any Brønsted acid. Subsequently, UV light was successfully used to change the shape of the vitrimer by locally activating the photocatalyst. The authors anticipate that this new spatial control technology enables the fabrication of sophisticated soft active devices that can change shape in a programmable manner. We look forward to reading more about such fantastic development from the Schlögl group.

Tips/comments directly from the authors:

• Owing to their strong Brønsted acidity and high thermal stability, photoacid generators are able to catalyze thermo-activated transesterifications in hydroxyl ester networks.
• Stress relaxation kinetics increase with rising catalyst content and rising irradiation dose.
• Since activation of the photoacid generator and the curing of the network can be achieved simultaneously by irradiating the desired layers with UV-A light (365 nm), a compromise between sufficient activation and resolution has to be made.
• Prior to the shape memory experiments it is important to thermally anneal the networks to form additional crosslink sites by hydrogen bonding, which leads to a change in thermal and mechanical properties. After 4 h at 140 °C, the network properties remain constant and the printed test specimen are able to repeatedly undergo shape changes after the programming step.
• Photoacid generators are highly versatile transesterification catalysts and can be applied for imparting dynamic network properties in numerous photopolymer systems. Network architecture can be conveniently adjusted by the structure and functionality of the monomers and/or crosslinkers.

Citation to the paper: Locally controlling dynamic exchange reactions in 3D printed thiol-acrylate vitrimers using dual-wavelength digital light processing, Polym. Chem., 2021,12, 3077-3083, DOI: 10.1039/d1py00427a

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d1py00427a 

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

Ntetsikas et al. synthesised a series of linear and miktoarm star copolymers to study their self-assembly behaviour in bulk.

Block copolymers consisting of high 1,4-microstructure-content polybutadiene (PB_1,4) blocks  and high 3,4-content polyisoprene (PB_1,4–b-PI_3,4) blocks self-assemble, due to their incompatibility, to form different nanostructures useful for various applications, such as electronic devices, nanotechnology and optoelectronics. In this work, Avgeropoulos and co-workers report a new synthetic procedure for the preparation of four linear PB_1,4–b-PI_3,4 diblock copolymers and eight asymmetric miktoarm star copolymers and investigate the effect of the architecture (linear versus non-linear) on microphase separation and final nanostructure of these copolymers. Furthermore, the results of this study have been compared with the PS(PI_1,4)_n (PS: polystyrene) well studied and established systems.

In particular, the authors combined anionic polymerization and selective chlorosilane chemistry to prepare four different sets of linear and star copolymers. Each set included one linear diblock copolymer with similar molecular characteristics to the corresponding PB_1,4(PI_3,4)₂ and PB_1,4(PI_3,4)₃ miktoarm stars. All copolymers were carefully characterized by size-exclusion chromatography (SEC), membrane osmometry (MO) and nuclear magnetic resonance (NMR) indicating a high degree of molecular and compositional homogeneity. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) were used to verify microphase separation and reveal the effect of the architecture on the adopted topologies. Through such a comprehensive characterization the authors have discovered that the high chain flexibility provided by the two polydiene segments affords promising properties previously unattainable from the corresponding triblock copolymers of these polydienes with polystyrene.

This work paves the way for further studies of material properties such as rheology and binary blends of the pure linear and non-linear copolymers with corresponding homopolymers (either hPB_1,4 or hPI_3,4).

We look forward to further exciting findings from the Avgeropoulos’ group.

Tips/comments directly from the authors:

• Morphological characterization studies reveal the coherence of theoretical studies on the PS(PI_1,4)_n system and the experimental results of the PB_1,4(PI_3,4)_n system (PS is substituted by PI_3,4).
• The only discrepancies from the relevant PS/PI system were found for two linear copolymers, where in both samples, hcp cylinders of the minority phase in the matrix of the majority were observed, instead of the expected DG cubic structure morphology.
• The almost identical electron densities between the two polydienes led to impossible morphological characterization through small angle X-ray scattering (SAXS) and only transmission electron microscopy results verify the adopted morphology for each copolymer.
• The adopted well-ordered nanostructures lead to the assumption that the segment–segment interaction parameter between the two polydienes of high 1,4-microstructure (∼92%) for the PB and ∼55–60% 3,4-microstructure for the PI is well above zero.
• It was really exciting to verify that if the 3,4-microstructure for the PI blocks was not within the regime of ∼55–60% then a homogeneous structure was adopted (no microphase separation).
• This regime of ∼55–60% 3,4-microstructure for the PI segments can be achieved by just adding a very small amount of a polar additive (∼1ml of THF) in the polymerization solvent ( 200 ml of benzene).

Citation to the paper: Synthesis, characterization and self-assembly of linear and miktoarm star copolymers of exclusively immiscible polydienes, Polym. Chem., 2021,12, 2712-2721, DOI: 10.1039/D1PY00258A

Link to the paper:

https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00258a#!divAbstract

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

Zhang et al. demonstrate a simple methodology for the synthesis of mechanically tough and self-healing materials.

Polymers that simultaneously exhibit high mechanical toughness and self-healing properties are essential to develop the next generation of materials for various applications including optoelectronics, sensors and healthcare devices. In this work, He, Li and co-workers developed a new route for the synthesis of a tough and self-healing supramolecular network by introducing Al(III)-carboxyl complexes into photo-polymerizable deep eutectic solvent. The resulting elastomers were mechanically tough and self-healing and demonstrated high transparency and stretchability as well as ionic conductivity. In particular, thanks to the synergic interaction of H-bonds and coordination bonds within the polymer matrix, over 80% self-healing efficiency for the damaged film could be obtained after 72 hours at ambient temperature. At the same time, the interaction between the fractures interfaces allows for the quick recovery of ion channels, therefore inducing electrical self-healing properties. The materials have been thoroughly characterized by a number of techniques including nuclear magnetic resonance, electrochemical impedance spectroscopy, differential scanning calorimetry, dynamic mechanical analysis, and tensile experiments. The developed methodology is simple, environmentally friendly and fast. The authors envision that their fundamental design concept will significantly contribute to the development of the next generation of tough and self-healing materials for potential use in flexible electronics and other applications.

Tips/comments directly from the authors:

• Although in this paper we only report on the preparation of transparent conductive polymers from Al(III)-based polymerizable deep eutectic solvents (PDESs), this method can also be extended to other types of metal cations such as copper, zinc, cobalt and nickel ions. However, considering that some metal ions are coloured, the prepared conductive elastomers may be suitable for other areas of application.
• During the experiments, we also found that PDES with trivalent iron ions could not be polymerized under UV light. The reasons for this phenomenon are not yet clear and we will continue to investigate.
• The prepared transparent conductive elastomers are somewhat hygroscopic and therefore need to be kept dry during storage and use to maintain their performance stability.
• If a high transparency film is desired, a release film should be necessary to insulate the polymer from oxygen during polymerization to maintain the homogeneity of the polymer structure.

Citation to the paper: Mechanically tough yet self-healing transparent conductive elastomers obtained using a synergic dual cross-linking strategy, Polym. Chem., 2021,12, 2016-2023, DOI:10.1039/D0PY01760D

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01760d 

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

Forsythe and Maynard present a new strategy for the direct synthesis of disulphide-bridging trehalose polymers using a bis-sulfone ATRP initiator.

Conjugated polymers are often used to enhance the properties of therapeutic molecules such as antibodies and antigen binding fragments (Fabs). In this work, Forsythe and Maynard present a new strategy to prepare polymer-antibody/Fab conjugates by employing bis-sulfone end-groups installed via a functionalized atom transfer radical polymerization initiator. In particular, a bis-sulfone initiator was first synthesized and subsequently subjected to activators generated electron transfer (AGET) polymerization using ascorbic acid as the reducing agent. Upon optimizing the ligand concentration (special care was taken here as an excess of ligand causes a detrimental side reaction which results in broadening of the molecular weight distributions), disulphide-reactive trehalose polymers could be efficiently prepared with controlled molecular weight and fairly low dispersity, thus confirming a controlled polymerization. The polymers were then conjugated to a full Immunoglobulin G and its Fab fragment and the reaction proceeded quantitatively as confirmed by western blot and mass spectrometry. The stability of the resulting conjugates was then assessed by accelerated heat stress studies where the trehalose polymer was found to considerably increase the thermal stability of both Herceptin and Herceptin Fab. Importantly, this new strategy allows for a facile way to synthesize polymeric bioconjugates without the need for time consuming post-polymerization modification methods while also exhibiting very good monomer compatibility. As the authors conclude, they anticipate a continued exploration in the field of antibody and protein conjugation and we look forward to reading the next exciting findings from the Maynard group.

Tips/comments directly from the authors:

• The bis-sulfone functionality is a robust system for the production of protein-polymer conjugates. However, due to its base-sensitivity, care needs to be taken when incorporating it into polymers. Since common ligands for AGET ATRP display reactivity towards the bis-sulfone, both reaction temperature and concentration should be kept low.
• For performing conjugations using the bis-sulfone, we found it important to do a two-step reduction and alkylation where the reducing agent (DTT) was removed prior to the conjugation. To help avoid re-oxidation of the disulfides during this process, we used a buffer containing EDTA to prevent trace metal-mediated oxidation and additionally used desalting columns that allow for rapid removal of excess DTT.
• While we demonstrate the applicability of the bis-sulfone initiator for AGET ATRP, the chemistry should be amenable to other controlled polymerizations such as RAFT. Incorporation into a chain transfer agent could further expand the diversity of chemistry available for antibody conjugation.
• Trehalose polymers stabilized the antibody and Fab to temperature increases. However, the same polymer could also increase the stability of the conjugates in vivo since these polymers have improved the pharmacokinetics of other proteins.

Citation to the paper: Synthesis of disulfide-bridging trehalose polymers for antibody and Fab conjugation using a bis-sulfone ATRP initiator, Polym. Chem., 2021,12, 1217-1223, DOI: 10.1039/D0PY01579B

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01579b 

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

Rossegger et al. present a new transesterification catalyst which can be applied in thiol-acrylate vitrimer systems enabling the fabrication of precise 3D objects.

Vitrimers are a unique class of materials that possess the remarkable property to be thermally processed in a liquid state while maintaining their network integrity. This property is induced owing to various thermo-activated exchange reactions including the catalyzed transesterification of hydroxyl ester moieties. However, the possibility to introduce dynamic covalent bonds into 3D printable photopolymers is challenging and the printed objects often suffer from a range of limitations such as low resolution, poor surface quality and lack of versatility. In addition, conventional transesterification catalysts exhibit poor solubility and present additional compromises on cure rate and pot life of photocurable resins. To this end, Schlögl and co-workers introduced a mono-functional oligomeric methacrylate phosphate as a new and efficient transesterification catalyst. The catalyst has many advantageous characteristics: it is liquid, easily dissolved in a range of acrylic monomers and can be covalently incorporated into the network across its methacrylate group. Once photo-cured, the dynamic thiol-click networks are able to rapidly undergo thermo-activated rearrangements of their network topology as shown by stress relaxation experiments. Importantly, when applied in thiol-acrylate vitrimer systems, precise 3D objects with 500 µm features using bottom-up digital light processing can be obtained. When compared to other commonly employed catalysts, the mono-functional methacrylate phosphate is superior both in terms of solubility and stress relaxation, thus unlocking a new toolbox of photocurable vitrimers.

Tips/comments directly from the authors:

• Owing to their strong Brønsted acidity, organic phosphates are able to catalyze transesterifications in hydroxyl ester networks. They exhibit a better performance in catalyzing exchange reactions in dynamic photopolymers compared to Lewis acids such as Zn(OAc)₂.
• Stress relaxation kinetics increase with rising catalyst content. However, the catalyst content should not exceed 50 mol% as the resin formulation is getting destabilized. Below 50 mol%, the thiol-click resin is stable over several weeks. This is a clear advantage compared to conventional transesterification catalysts, which initiate thiol-Michael reactions and lead to a premature gelation of thiol-click resins.
• Prior to the shape memory experiments it is important to thermally anneal the networks to form additional crosslink sites by hydrogen bonding, which lead to a change in thermal and mechanical properties. After 4 h at 180 °C, the network properties remain constant and the printed test specimen are able to repeatedly undergo shape changes after the programming step.
• Organic phosphates are highly versatile transesterification catalysts and can be applied for imparting dynamic network properties in numerous photopolymer systems. Network architecture can be conveniently adjusted by the structure and functionality of the monomers and/or crosslinkers.

Citation to the paper: Digital light processing 3D printing with thiol–acrylate vitrimers, Polym. Chem., 2021,12, 639-644, DOI: 10.1039/D0PY01520B

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01520b

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.